In a cellular network, for example a UMTS (“Universal Mobile Telecommunication System”) type or a 3GPP (“3rd Generation Partnership Project”) type cellular network, equipment must be synchronised on a cell before it can transmit and/or receive data quickly. Therefore, in order to optimise information transmissions from or to terminals, it is necessary to enable fast synchronisation of terminals on a cell in the network.
Synchronisation techniques are known based on the use of a dedicated channel, for example of the SCH (“Synchronisation CHannel”) type. This synchronisation is usually relatively effective but it has the disadvantage that it is slow under some circumstances, particularly when the equipment moves from one cell to another and changes frequency.
Furthermore, in order to take account of the mobility of equipment in a cellular network without interrupting communications in progress, the network must manage cell transfers or “hand-overs” that enable equipment to change from cell to cell very quickly. Note that a “hand-over” is used particularly when it is preferable for terminal in communication with a fixed station (Base Station BS responsible for the management of a cell) to change the base station with which it is associated, to communicate. A “hand-over” can be used for different reasons: bad quality of radio links, saturation of a base station due to too many communications, mobility of the equipment that moves away from a first station and towards another station, better adaptation of a base station for the service requested by the terminal, etc.
This “hand-over” must be made quickly, for reasons of efficiency, quality and fluidity of the service offered to the user.
Conventionally, a relatively slow inter-frequency “hand-over” procedure is used (in other words when the frequencies of cells to which equipment is attached are different before and after the “hand-over”) which generates interference.
In order to make an inter-frequency “hand-over” in a third generation radiotelephony system using a channel access technique based on a CDMA (Code Division Multiple Access), for example in a UMTS or 3GPP system, a mobile equipment has to use vacant periods in “compressed mode” to synchronise itself onto different cells in order to make power measurements on these cells. For synchronisation, the mobile equipment must listen to the primary SCH channel, then the secondary SCH channel, then the CPICH channel on which it makes the power measurement.
This technique according to prior art has the disadvantage that it has a very long synchronisation phase since vacant periods output from compressed mode are short duration.
Therefore the UMTS and 3GPP networks are defined such that before initialising a communication, the mobile equipment is already on an optimum cell which avoids the need for a “hand-over” at the beginning of a communication.
The various aspects of the invention are intended particularly to overcome these disadvantages in prior art.
More precisely, a first purpose of the invention is to enable fast synchronisation when equipment moves from one cell to another.
Another purpose of the invention is to enable a fast “hand-over” of equipment particularly in the case of an inter-frequency “hand-over” procedure.
Another purpose of the invention is to enable the use of equipment intended for third generation mobile communication networks, requiring no or few modifications to current standards in force and particularly the UMTS FDD (Frequency Division Duplex) standard (in particular, series 25 of this standard) defined and distributed by the 3GPP (“3rd Generation Partnership Project”) committee.